A surface aerator generally comprises a power drive fitted on a float or a rigid structure of a purification plant. The motor drives a shaft provided with impeller means such as a paddle wheel or a propeller generally combined with a static diffusion or deflection head. There are several kinds of aerators among which one can make a distinction between low speed aerators (.+-.60 rev/min) and high speed aerators (700-1800 rev/min).
A low speed aerator mainly comprises an impeller shaped as a conical paddle wheel having a large diameter, a part of which extends below the surface level of the water and which is driven by an electric motor through a speed-reducing gear. The efficiency of such aerators is very good in clean water.
High speed aerators are provided with a propeller shaft directly connected to the motor. The water is forced upwardly at high speed by the screw-propeller and is diffused horizontally by a static deflector or by a trailing edge. According to their application, these aerators have more or less good efficiency in water loaded with low amounts of sludge.